Coral Reefs and Corals

Reefs are found in the fossil record and are thought to be about two billion years old. These reefs were built by calcareous algae and not corals. The first true corals did not appear until about 300–500 million years ago. They apparently flourished until a devastating extinction occurred killing many groups of corals. Most of the corals that compose extant (still living) reefs were found around 65 million years ago. They are still a vital part of the living environment. In addition to their ecological role as a foundation for a wide variety of life forms, the coral reefs have become a haven for tourists and scuba divers.

Corals are animals that belong to the monophyletic group called the Cnidaria (formerly called a Phylum). They are named for specialized stinging structures that emit long, venomous barbs when stimulated by the presence of prey or danger. Corals are further classified into a smaller subdivision, the Anthozoa. For geologists, the most important characteristic of many corals is their ability to remove calcium from the water and redeposit it as a hardened outer casing in the form of calcium carbonate. Because many corals species are colonial, the chambers of the animals grow together forming a larger hardened structure. Over many years, these constructions may merge with others of the same species or with those of different species. The resulting large and diverse colony makes an impressive and rigid structure that spreads across the sea bottom. At this point, it is identified as a reef. Within the reef, there are hiding places for fish and other marine organisms. Organic material is abundant and many species live their entire lives within the confines of the protection the reef provides.

Corals are believed to live in a symbiotic relationship with species of microscopic algae. As a result, reef corals are rarely found below the photic zone or about 150 feet (46 m). The photic zone is a narrow strip of surface water (about 200 feet) through which sunlight permeates. Below this depth particles in the water prevent light waves from penetrating.

Corals require clear water to flourish. They need as much light energy as possible so the algae in their tissues can thrive. In addition, they prefer areas where wave energy is high. The waves bring floating organic materials, oxygen, and nutrients to the corals. Reef-building corals require warm ocean temperatures (68 to 82°F, or 20 to 28°C). Because of this requirement, corals are primarily confined to areas within 30°N and 30°S latitudes. This region includes the tropical and subtropical Western Atlantic and Indo-Pacific Oceans. Western Atlantic reefs are found in Bermuda, the Bahamas, the Caribbean Islands, Belize, Florida, and the Gulf of Mexico. The Indo-Pacific ocean region extends from the Red Sea and the Persian Gulf through the Indian and Pacific oceans. Reefs have been found as far as the western coast of Panama. The rocky outcrops in some areas of the Gulf of California are also favorable regions for corals. These warm shallow seas provide ideal conditions where water temperatures and salinity are high and carbon dioxide concentrations are low. In this type of habitat the corals are able to precipitate calcium out of the water.

The colors of the reefs are impressive and add to their beauty. The United States National Oceanographic and Atmospheric Administration describe natural pigments in coral tissue that produce a range of colors including white, red, orange, yellow, green, blue, and purple, along with algae that live within the tissues of some corals that may make the coral appear brown, green, or orange. Unfortunately, this characteristic of some corals has made them prized for their use in jewelry. Removal of living corals for such uses has put many species in danger of extinction or severe harm.

The massive structure of some reefs is especially important to geologists because the colonies can reshape coastal sedimentation and deposition regimes. Barrier islands provide some of the more spectacular examples of this. As seamounts grow in warm oceans corals land on the emerging rock shore and begin their colonies. Over millions of years, the islands may move and sink because of plate tectonics. The corals continue to grow on top of one another as they attempt to stay near the surface. The reef expands until large areas of coastline are bordered by the reefs. Waves are slowed by the reefs and any sediment they carry is dropped on the ocean side of the reef. The water that flows over the coral and toward the beach has little energy and is relatively clear. Any sediment it carries is fine-grained and deposited in a space called a lagoon. Lagoons are warm bodies of water that lie between the reef and shore. The waters are calm and warm. Numerous species of plants and animals live in a lagoon. They are favored locations of tourists for swimming and snorkeling and provide ideal spots for recreation.

The continuing cycle of growth of the corals and deposition of sediments are well documented in the fossil record. Reefs sometimes grow to a massive size, and are often identified in rocks such as those found in the Big Bend National Park in Texas. The El Capitan Reef is an exceptional example of such a structure. This is one of the types of evidence geologists use to reconstruct climate in various regions of the globe.

Coral reefs are disappearing in many places around Earth. Projects, such as those in Florida where old ships are sunk to provide new surface area for coral colonies to grow, are helping save the reefs from destruction by divers and fishing boats. Water pollution and disease still threaten many species. Without coral reefs, entire marine ecosystems may vanish, and an ancient geological and biological system might disappear from Earth.

See also Great Barrier Reef; Oceanography